1. Field of the Invention
This invention relates generally to subsea blowout preventers of the type used in the oil and gas drilling and production industry and, in particular, to an improved valve for venting gas through a wall of a subsea blowout preventer.
2. Description of the Background
It is well known that blowout preventers can be opened and closed to control the pressures in an earth wellbore. This is also known as either being in an unsealed (open) or in a sealed (closed) position. When a blowout preventer ("BOP") is used to control an oil or gas well that is drilled on land, it is possible to open the BOP to vent well gases under pressure that have built up in the wellbore. In subsea oil and gas operations, however, this is not possible with the subsea BOPs presently being used. The difficulty arises from the fact that the subsea BOP is situated on the sea floor and from the fact that there is often a significant length of casing from the subsea BOP on the sea floor to the drilling or production equipment mounted on the platform of the offshore rig. In the case of the drilling rig, the column of drilling mud that is present in the length of casing from the subsea BOP to the drilling platform on the rig prevents the safe and effective venting of wellbore gases that have become trapped in the annulus under the subsea BOP.
If one attempts to open and thus to vent the high pressure wellbore gases trapped under a subsea BOP through the column of drilling mud, one risks a high pressure, uncontrolled explosion of wellbore gases. The drilling mud acts as a viscous plug which unpredictably impedes the passage of the high pressure gases to the surface.
There has therefore arisen a need for a safe and effective means for venting high pressure wellbore gases without opening a subsea BOP. The present invention meets this need by providing a failsafe pressure release valve for subsea blowout preventers which is independent of whether the BOP is open.